For me, the scare-factor came from the relentless nature of the monsters... Sure, you could outrun one or two of them individually, but eventually through sheer numbers they overwhelmed you, dragged you down and then SLOWLY started to gnaw on you.

Somewhere in the early-2000s with movies like 28 Days Later and Dawn of the Dead, zombies were re-invented as these sprinting creatures that took you down like a pack of wolves. Which is why fast zombies don't scare me so much... because there really isn't any difference between them and a pack of, say, crazed weasels.

Plus the science geek inside me wonders how zombies could run with muscles that are decaying and decomposing. Slow zombies are consistent with the idea that their muscles are atrophying and no longer function efficiently.